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Besides being exact, “the experiment should be multiplied and varied so that it will appear for certain that the facts do not flow from fortuitous circumstances, but constantly from the nature of individuals, and, therefore, there may be had the right of inferring a universal law.” The astronomer must make his observations at the various phases of the phenomenon; the medical man must try his drug on old and young, in one stage of the disease and then in another. The chemist, the physicist, all must vary their experiments, perform a number of them, and, where there is a discrepancy, take into account the average. Let us suppose •that the statistician concludes the death rate of a certain locality by taking into account the number of deaths during a year in which an epidemic raged in that section. We can readily see how far he is from telling the true {."ate of the affair. He must take an average of many, say all, the months, and of a number of consecutive years, if he wishes to know, the death rate of that locality as compared with other places. . Taking into account these various examples, we may conclude that not one incident, but an average of various incidents, will enable us to approximate truth.
Though attention to the minutest details, as well as variety of experimental work in every form, may go a great way toward arriving at a Scientific Induction, there still remains a third aid which must not be overlooked. The mind exhibits a constant demand for explanation. If, on entering a room, I find a plant upset on the floor, I may conclude that a cat has knocked it over. If, upon examination, I find no cat in the room, discover the windows screened, and remember that the door had been found closed, I must abandon my notion and find a new cause for the effect. Perhaps, the wind blew it over. Again I cast about to see if the evidences fit the new conjecture. I hold or reject the conjecture, according as the evidences point or fail to point to the possibility that the conjecture was correct. This shows how natural it is to make some hypothesis. As in our everyday life, so in science, we build and, when necessary, tear down the hypothesis.
The hypothesis is simply a supposition. If the supposition is found to work, and newly discovered facts fit the hypothesis, we dignify it by the name of theory. If the theory becomes generally accepted, and seems to be the true solution of a question, it becomes a universally accepted law. Laplace conceived an idea for the evolution of the planetary system. Nothing in the idea seems opposed to reason. It holds its own, though many other theories have been advanced. Provided no more plausible explanation is found, the theory of Laplace is likely to become a law. The hypothesis, moreover, must be fruitful. Three of its uses stand out as having been of great service in the past: it serves as the foundation of a theory which ultimately may become a law; it points out the way for a true theory; it links together facts already observed. As an example of the first, we may cite the theory of Evolution, which, as far as plants and animals are concerned, is now pretty generally accepted. The law of multiple proportion also furnishes an example. Kepler’s case serves as an illustration of the second use. After formulating twenty-nine different theories concerning the movement of the heavenly bodies, he rejected one after another until he formulated the present theory. The Evolution theory may serve as an instance of the third use. Even those who do not accept the theory in the biological world use it as a working theory. The atomic theory is another example

Besides being exact, “the experiment should be multiplied and varied so that it will appear for certain that the facts do not flow from fortuitous circumstances, but constantly from the nature of individuals, and, therefore, there may be had the right of inferring a universal law.” The astronomer must make his observations at the various phases of the phenomenon; the medical man must try his drug on old and young, in one stage of the disease and then in another. The chemist, the physicist, all must vary their experiments, perform a number of them, and, where there is a discrepancy, take into account the average. Let us suppose •that the statistician concludes the death rate of a certain locality by taking into account the number of deaths during a year in which an epidemic raged in that section. We can readily see how far he is from telling the true {."ate of the affair. He must take an average of many, say all, the months, and of a number of consecutive years, if he wishes to know, the death rate of that locality as compared with other places. . Taking into account these various examples, we may conclude that not one incident, but an average of various incidents, will enable us to approximate truth.
Though attention to the minutest details, as well as variety of experimental work in every form, may go a great way toward arriving at a Scientific Induction, there still remains a third aid which must not be overlooked. The mind exhibits a constant demand for explanation. If, on entering a room, I find a plant upset on the floor, I may conclude that a cat has knocked it over. If, upon examination, I find no cat in the room, discover the windows screened, and remember that the door had been found closed, I must abandon my notion and find a new cause for the effect. Perhaps, the wind blew it over. Again I cast about to see if the evidences fit the new conjecture. I hold or reject the conjecture, according as the evidences point or fail to point to the possibility that the conjecture was correct. This shows how natural it is to make some hypothesis. As in our everyday life, so in science, we build and, when necessary, tear down the hypothesis.
The hypothesis is simply a supposition. If the supposition is found to work, and newly discovered facts fit the hypothesis, we dignify it by the name of theory. If the theory becomes generally accepted, and seems to be the true solution of a question, it becomes a universally accepted law. Laplace conceived an idea for the evolution of the planetary system. Nothing in the idea seems opposed to reason. It holds its own, though many other theories have been advanced. Provided no more plausible explanation is found, the theory of Laplace is likely to become a law. The hypothesis, moreover, must be fruitful. Three of its uses stand out as having been of great service in the past: it serves as the foundation of a theory which ultimately may become a law; it points out the way for a true theory; it links together facts already observed. As an example of the first, we may cite the theory of Evolution, which, as far as plants and animals are concerned, is now pretty generally accepted. The law of multiple proportion also furnishes an example. Kepler’s case serves as an illustration of the second use. After formulating twenty-nine different theories concerning the movement of the heavenly bodies, he rejected one after another until he formulated the present theory. The Evolution theory may serve as an instance of the third use. Even those who do not accept the theory in the biological world use it as a working theory. The atomic theory is another example